A clear cut relationship has been established in E. coli between the presence of K1 antigen and ability to produce neonatal meningitis. Epidemiological studies and our own preliminary data suggest that other factors besides K1 antigen play a role in E. coli K1 pathogenicity, i. e.: Col V, Hemolysin. Using bacterial genetic and molecular cloning techniques E. coli strains will be constructed carrying K1 antigen and other determinants related to pathogenicity i.e, Col V, hemolysin. These strains will be tested for pathogenicity in mice and in 5 day old rats, where increase or decrease of pathogenic potential can be related to the occurrence or not, of one or more determinants. Experiments will be directed to investigate the contribution of K1 antigen and other determinants to the overall E. coli K1 pathogenicity. Information is such a way obtained will be relevant to current efforts to find ways to prevent E. coli K1 disease in the newborn. It will also give information useful to understand E. coli pathogenicity as a whole, knowledge relevant to the recombinant DNA field where the host for recombinant DNA molecules is E. coli K12.